In order to induce a therapeutic immunomodulatory activity, 11 patients with high-grade non-Hodgkin's lymphoma (HG-NHL) at a median of 42 days after autologous bone marrow transplantation (ABMT) received recombinant interleukin-2 (rIL-2) subcutaneously at a dose of 2 international megaunits (IMU)/m2 every other day for 2 weeks and then 3 IMU/m2 twice a week for 1 year. Immunological studies, including T and natural killer (NK) cell subset assessment, together with functional assay, such as NK and CD16-mediated cytotoxic activities, were performed before therapy, after 2 weeks and then monthly. Phenotypic analyses showed a significant and persistant (P = 0.001) increase in the proportion and absolute number of total lymphocytes and, particularly of both CD16 and CD56 NK cells, from pre-treatment values of 14 and 18% to 30 and 38% respectively, recorded after 6 months of therapy. No changes were observed in CD25 (p55)-positive cells, while a significant increase from 13 to 33% (after 6 months) was observed in CD122-positive cells. Furthermore, rIL-2 administration led to an enhancement of NK activity even at the lowest effector:target ratio and of CD16-mediated cytotoxic activity. Clinical tolerance was acceptable with moderate fever and fluid retention observed only at the onset of rIL-2 treatment. None of the patients have progressed with a median follow-up of 22 months (range 10-42 months) after starting therapy. In addition, two patients with a residual disease after ABMT, one in the liver and the second in the lymph nodes, obtained a complete response after 10 and 7 months of rIL-2 therapy, respectively. These preliminary data suggest that the infusion of low-dose rIL-2 s.c. after ABMT is safe and well tolerated and can selectively increase the NK cell number and function. Additional patients are needed in order to assess the impact of these immunological changes on relapse-free survival after ABMT for HG-NHL.